Ganymede should appear approximately that wide from Europa when they are closest. According to my rough calculations Io can appear as wide as 50 minutes wide in the sky of Europa, about one and two thirds the apparent width of the Moon as seen from Earth. Io has a diameter of about 3,650 kilometers, and can be as close as about 249,000 kilometers to Europa. Of course Europa would be on the far side of Jupiter and hidden from Callisto anyway. At its farthest from Callisto it is about 2,553,743 kilometers from Callisto, and according to my rough calculations should appear to be about 4.2 arc minutes wide from Callisto, about an eighth of the diameter of the Moon as seen from Earth. When they are farthest apart they are on opposite sides of Jupiter and hidden by it as well.Įuropa, with a diameter of 3,121.6 kilometers, is the smallest of the Galilean moons. So the distance between Io and Europa varies from about 249,000 to 1,092,734 kilometers. The orbit of Io has a semi-major axis or radius of 421,700 kilometers, Europa's orbit has a radius of 671,034 kilometers, Ganymede's 1,070,412 kilometers, and Callisto's 1,882,709 kilometers. Would it be possible to see one from the other with the naked eye if they were aligned?īecause Io and Europa orbit Jupiter in different orbits, the distance between them is not constant. They seem to be about 250,000 kilometers away from each other. The two moons that are closest to each other are Io and Europa. And if a moon was larger than the smallest possible rounded moon, it would appear as disc in the sky of another moon even if the distance between them was more than about 5,729,582.7 kilometers, or 3,560,197.6 miles. So whenever a smallest possible rounded moon was closer than about 5,729,582.7 kilometers, or 3,560,197.6 miles, it would definately appear as a disc. I may also point out that among those moonsof a giant plaent which are large and massive enough to be gravitationally rounded, the smallest moon with a rounded form would appear on the borderline between a dot of light and a disc with a visible diameter when it was about 5,729,582.7 kilometers, or 3,560,197.6 miles from another moon. The author of that article calculates the inner and outer limits, in terms of radii of their planets, for the orbits of habitable exomoons.Īnd from that range of orbital distances I roughly calculated that a habitable exomoon would orbit at distances such that the giant exoplanet that it orbits would appear to be between 5.7295 to 22.9183 degrees wide, about 11 to 45 times the angular diameter of the Moon. In my answer to this question What do I bear in mind, creating an earth-like world with TWO moons? the last part of my answer links to this scientific article discussing exomoon habitabilty: No human missions are planned as yet, due to the extreme radiation environment and highly toxic atmosphere and surface.Science fiction writers and scientists have imagined the possibility of exomoons orbiting giant exoplanets in other star systems being habitable.Īnd there have been scientific articles discussing the possible limits of exomoon habitabilty. Robotic missions to Io could study its volcanism in closer detail. The volcano has a staggering diameter of 126 miles (202 km), but more incredible than its size is the molten underground lava lake feeding Loki its material.Io is made mostly of silicate rocks, and its surface is painted with sulfur particles from the volcanoes and frosts that are created as the atmospheric gases freeze out and fall to the ground.The average height of Io’s peaks are around 6 km. Io has a number of mountains, some of which rise up as high as Mount Everest on Earth.The volcanic plumes of Io rise up as high as 200 km, showering the terrain with sulfur, sulfur dioxide particles, and rocky ash.Some of the material becomes part of a ring of charged particles around Jupiter called the Io plasma torus. Gases from the atmosphere escape to space at the rate of about a ton per second. Io has a very thin atmosphere that contains mostly sulfur dioxide (emitted from its volcanoes).The volcanoes of Io are constantly erupting, creating plumes that rise above the surface and lakes that cover vast areas of the landscape.The volcanism on Io is due to tidal heating, as the moon is stretched by Jupiter’s strong gravitational pull and by the lesser gravitational effects of the other satellites.They make this little moon the most actively volcanic world in the solar system. Io has more than 400 active volcanoes on its surface.Jupiter’s Galilean moons size comparison ( Io, Europa, Ganymede and Callisto) Facts about Io
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